Hydrothermal synthesis has emerged as a powerful method for producing high-quality nanocrystals with controlled morphology, crystallinity, and composition. Among these, perovskite-type barium titanate (BaTiO3) nanocrystals have gained attention as active materials in triboelectric nanogenerators (TENGs) due to their high dielectric properties and inherent polarization characteristics. Unlike piezoelectric nanomaterials, which rely on strain-induced polarization, triboelectric materials generate charge through contact electrification and electrostatic induction, where surface polarization and dielectric properties play a critical role.
The hydrothermal method enables precise control over BaTiO3 nanocrystal size, phase purity, and surface chemistry, which are crucial for optimizing triboelectric performance. By adjusting reaction parameters such as temperature, precursor concentration, and reaction time, single-domain ferroelectric nanocrystals with enhanced spontaneous polarization can be synthesized. For instance, hydrothermal reactions conducted at 200°C for 12–24 hours yield BaTiO3 nanocrystals with sizes ranging from 50 to 200 nm, exhibiting a tetragonal phase that is essential for high polarization. The presence of hydroxyl groups on the nanocrystal surface further enhances charge trapping, which is beneficial for triboelectric charge retention.
Polarization enhancement in BaTiO3 nanocrystals is achieved through several strategies. Doping with transition metals such as cobalt or iron introduces defect dipoles that align with the ferroelectric domains, increasing the effective polarization. Studies have shown that Co-doped BaTiO3 nanocrystals exhibit a 30% increase in dielectric constant compared to undoped samples, directly improving triboelectric charge density. Another approach involves creating core-shell structures, where a high-dielectric shell (e.g., SiO2 or TiO2) encapsulates the BaTiO3 core, preventing charge leakage and enhancing surface charge density. Additionally, poling under an external electric field aligns the ferroelectric domains, further amplifying the polarization effect.
Integration of hydrothermal BaTiO3 nanocrystals into TENGs requires careful consideration of device architecture and material compatibility. A common configuration involves dispersing the nanocrystals in a polymer matrix such as polydimethylsiloxane (PDMS) or polyvinylidene fluoride (PVDF) to form a composite triboelectric layer. The high dielectric constant of BaTiO3 increases the effective surface charge density of the composite, while the polymer matrix provides mechanical flexibility. Optimal loading concentrations typically range between 5–15 wt%, as higher concentrations may lead to aggregation and reduced film uniformity.
Device performance is also influenced by the contact-separation mode and electrode design. Vertical contact-separation TENGs with BaTiO3-PDMS composite films have demonstrated output voltages exceeding 200 V and power densities of up to 3 W/m² under optimized conditions. The use of interdigitated electrodes or microstructured surfaces further enhances charge transfer efficiency by increasing the contact area. Surface functionalization of BaTiO3 nanocrystals with hydrophobic ligands (e.g., oleic acid) can reduce humidity-induced charge dissipation, improving environmental stability.
A critical advantage of hydrothermal BaTiO3 nanocrystals in TENGs is their stability under repeated mechanical stress. Unlike piezoelectric materials, which may suffer from fatigue due to cyclic strain, triboelectric materials rely on surface effects, making them more durable for long-term operation. Furthermore, the hydrothermal synthesis route is scalable and environmentally benign, avoiding the need for high-temperature calcination or toxic solvents.
Future developments may focus on hybrid systems where BaTiO3 nanocrystals are combined with conductive nanomaterials (e.g., carbon nanotubes or graphene) to further enhance charge transport. Additionally, exploring new dopants and surface modifications could unlock higher triboelectric performance. The versatility of hydrothermal synthesis allows for tailoring nanocrystal properties to meet specific TENG requirements, paving the way for next-generation energy harvesting devices.
In summary, hydrothermal BaTiO3 nanocrystals offer a promising pathway for advancing triboelectric nanogenerators through their tunable polarization and dielectric properties. By leveraging doping, core-shell engineering, and optimized device integration, these materials can significantly improve TENG efficiency and reliability, opening new opportunities for self-powered electronics and wearable energy harvesters.